This invention relates to copper alloys having satisfactory strength, electrical conductivity and stress relaxation characteristics that are suitable for use as materials for connectors and other electrical or electronic components, as well as small Young""s modulus.
With the recent advances in electronics, the wire harnessing in various machines has increased in the degree of complexity and integration and this in turn has led to the growth of wrought copper materials for use in connectors and other electrical or electronic components.
The demands required of materials for connectors and other electrical or electronic components include
lightweightness, high reliability and low cost. To meet these requirements, copper alloy materials for connectors are becoming smaller in thickness and in order to press them into complex shapes, they must have high strength and elasticity, as well as good electrical conductivity and press formability.
Specifically, electrical terminals must have sufficient strength that they will not buckle or deform during connection and disconnection or upon bending, as well as sufficient strength to withstand caulking of electrical wires and connector fitting followed by holding in position. To meet this need, electrical materials for use as terminals are required to have a 0.2% yield strength of at least 600 N/mm2, preferably at least 650 N/mm2, more preferably at least 700 N/mm2, and a tensile strength of at least 650 N/mm2, preferably at least 700 N/mm2, more preferably at least 750 N/mm2. In addition, in order to prevent chain transfer of deterioration that may occur during pressing, terminals must have sufficient strength in a direction perpendicular to that of working operations such as rolling. To meet this need, electrical materials for use as terminals are required to have a 0.2% yield strength of at least 650 N/mm2, preferably at least 700 N/mm2, more preferably at least 750 N/mm2 and a tensile strength of at least 700 N/mm2, preferably at least 750 N/mm2, more preferably at least 800 N/mm2, in the perpendicular direction.
Further, in order to suppress the generation of Joule""s heat due to current impression, electrical materials for use as terminals preferably have a conductivity of at least 20% IACS. Another requirement is that the materials have great enough Young""s modulus to ensure that connectors of small size can produce great stress in response to small displacement but this has increased rather than reduced the production cost of terminals because the need for closer dimensional tolerances has required rigorous control not only in mold technology and pressing operations but also over variations in the thickness of strip materials to be worked upon as well as the residual stress that develops in them. Under these circumstances, it has become necessary to design a structure that uses a strip material of small Young""s modulus and which undergoes a large enough displacement to allow for substantial dimensional variations. To meet this need, electrical materials for use as terminals are required to have a Young""s modulus of 120 kN/mm2 or less, preferably 115 kN/mm2 or less, in the direction where they were wrought and a Young""s modulus of 130 kN/mm2 or less, preferably 125 kN/mm2 or less, more preferably 120 kN/mm2 or less in the perpendicular direction.
The above situation has become complicated by the fact that the frequency of mold maintenance accounts for a substantial portion of the production cost. One of the major causes of mold maintenance is worn mold tools. Since mold tools such as punches, dies and strippers wear as a result of repeated punching, bending or other press working operations, burring and dimensional inaccuracy will occur in the workpiece. The effect of the material itself on the wear of mold tools is by no means negligible and there is a growing need to reduce the likelihood of the material for causing mold wear.
Connectors are required to have high resistance to corrosion and resistance to stress corrosion cracking. Since female terminals are subject to thermal loading, they must also have good anti-stress relaxation characteristics. Specifically, their stress corrosion cracking life must be at least three times as long as the value for the conventional class 1 (specified by Japanese Industrial Standard, or JIS) brass and their percent stress relaxation at 150xc2x0 C. must be no more than one half the value for the class 1 brass, typically 25% or less, preferably 20% or less and more preferably 15% or less.
Brasses and phosphor bronzes have heretofore been used as connector materials. The lower-cost brass, even if its temper grade is H08 (spring), has a yield strength (proof stress) and a tensile strength of about 570 N/mm2 and 640 N/mm2, respectively, thus failing to satisfy the above-mentioned minimum requirements for yield strength (xe2x89xa7600 N/mm2) and tensile strength (xe2x89xa7650 N/mm2). Brass is also poor not only in resistance to corrosion, resistance to stress corrosion cracking, but also in anti-stress relaxation characteristics. Phosphor bronze has good balance between strength, resistance to corrosion, resistance to stress corrosion cracking, and anti-stress relaxation characteristics; on the other hand, the electrical conductivity of phosphor bronze is small (12% IACS for spring phosphor bronze) and an economic disadvantage also results.
Many copper alloys have been developed and proposed to date with a view to solving the aforementioned problems. Most of them have various elements added in small amounts such that they keep in a balance between important characteristics such as strength, electrical conductivity and stress relaxation. However, their Young""s modulus was as high as 120-135 kN/mm2 in the direction where the alloy was wrought and in the range of 125-145 kN/mm2 in the perpendicular direction. In addition, their cost was high.
Under these circumstances, researchers are most recently having a new look at brass and phosphor bronze because they both have small enough Young""s moduli (110-120 kN/mm2 in the direction where the alloy is wrought and 115-130 kN/mm2 in the perpendicular direction) to meet the aforementioned design criteria. Thus, it is desired to develop a copper alloy that is available at a comparable price to brasses and which exhibits a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young""s modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction in which the alloy is wrought while exhibiting a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in the perpendicular direction.
Connector materials are given Sn plating in an increasing number of occasions and the usefulness of alloys is enhanced by incorporating Sn. Inclusion of Zn as in brasses increases the ease with which to produce alloys having a good balance between strength, workability and cost. From this viewpoint, Cuxe2x80x94Znxe2x80x94Sn alloys may well be worth attention and known examples are copper alloys having designations ranging from C40000 to C49900 that are specified by the CDA (Copper Development Association), U.S.A. For example, C42500 is a Cu-9.5Zn-2.0Sn-0.2P alloy and well known as a connector material. C43400 is a Cu-14Zn-0.7Sn alloy and used in switches, relays and terminals, though in small amounts. However, little use as connector materials is made of Cuxe2x80x94Znxe2x80x94Sn alloys having higher Zn contents. In other words, increased Zn and Sn contents lower hot workability and unless thermo-mechanical treatments are properly controlled, various characteristics such as the mechanical ones desired for the connector materials cannot be developed and, what is more, nothing has been known about the appropriate Zn and Sn contents and the conditions for producing the desired connector materials.
Specific examples of copper alloys containing more Zn than C42500 include C43500 (Cu-18Zn-0.9Sn), C44500 (Cu-28Zn-1Sn-0.05P) and C46700 (Cu-39Zn-0.8Sn-0.05P) and they are fabricated into sheets, rods, tubes and other shapes that only find use in musical instruments, ships and miscellaneous goods but not as wrought materials for connectors, particularly as strips. Even these materials fail to satisfy all requirements for connector materials, representative examples of which are as follows:
(1) that they have a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young""s modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where the alloy was wrought;
(2) that they have a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in a direction perpendicular to the one where the alloy was wrought;
(3) that they have good press formability; and
(4) that they have high resistance to stress corrosion cracking.
The present invention has been accomplished under these circumstances and has as an object providing a copper alloy for use as connectors that can be manufactured at low cost and which exhibits good performance in 0.2% yield strength, tensile strength, electrical conductivity, Young""s modulus, anti-stress relaxation characteristics, press formability and any other qualities that are currently required of materials for connectors and other electrical or electronic components in view of the recent advances in electronics.
Another object of the invention is to provide a process for producing such connector copper alloys.
As a result of the intensive studies they made in order to attain the above-stated objects, the present inventors found optimum proportions of Zn and Sn in the Cuxe2x80x94Znxe2x80x94Sn alloy that could simultaneously satisfy the above-mentioned characteristics required of materials for connectors and other electrical or electronic components. At the same time, they found that in order to implement those characteristics, the relationship between the conditions for cooling ingots and rolling them and the conditions for subsequent heat treatments was extremely important. Based on this finding, the present inventors set the optimum processing and working conditions, eventually accomplishing the present invention.
Thus, according to the first aspect of the invention, there is provided a connector copper alloy that contains 23-28 wt % Zn and 0.3-1.8 wt % Sn while satisfying the following relation (1), with the balance being Cu and incidental impurities:
6.0xe2x89xa60.25X+Yxe2x89xa68.5xe2x80x83xe2x80x83(1) 
where X is the addition of Zn (in wt %) and Y is the addition of Sn (in wt %), further characterized in that said alloy has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, an electrical conductivity of at least 20% IACS, a Young""s modulus of no more than 120 kN/mm2 and a percent stress relaxation of no more than 20%.
According to the first aspect of the invention, there is also provided a connector copper alloy that contains 23-28 wt % Zn and 0.3-1.8 wt % Sn while satisfying the following relation (1), with the balance being Cu and incidental impurities:
6.0xe2x89xa60.25X+Yxe2x89xa68.5xe2x80x83xe2x80x83(1) 
where X is the addition of Zn (in wt %) and Y is the addition of Sn (in wt %), further characterized in that said alloy has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young""s modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas it has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.
Either of the copper alloys described above may further contain at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-3 wt % Si, 0.01-5 wt % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.
According to the second aspect of the invention, there is provided a process for producing a connector copper alloy which comprises the steps of:
melting an alloy that contains 23-28 wt % Zn and 0.3-1.8 wt % Sn while satisfying the following relation (1), with the balance being Cu and incidental impurities:
6.0xe2x89xa60.25X+Yxe2x89xa68.5xe2x80x83xe2x80x83(1) 
where X is the addition of Zn (in wt %) and Y is the addition of Sn (in wt %);
cooling the melt from the liquidus line to 600xc2x0 C. at a rate of at least 50xc2x0 C./min; and
subsequently hot rolling the resulting ingot at an elevated temperature of 900xc2x0 C. or below.
According to the second aspect of the invention, there is also provided a process for producing a connector copper alloy which comprises the steps of:
melting an alloy that contains 23-28 wt % Zn and 0.3-1.8 wt % Sn while satisfying the following relation (1), with the balance being Cu and incidental impurities:
6.0xe2x89xa60.25X+Yxe2x89xa68.5xe2x80x83xe2x80x83(1) 
where X is the addition of Zn (in wt %) and Y is the addition of Sn (in wt %);
cooling the melt from the liquidus line to 600xc2x0 C. at a rate of at least 50xc2x0 C./min;
subsequently hot rolling the resulting ingot at an elevated temperature of 900xc2x0 C. or below; and
repeating the process of cold rolling and annealing in a temperature range of 300-650xc2x0 C. until the as-annealed rolled strip has a crystal grain size of no more than 25 xcexcm.
According to the second aspect of the invention, there is also provided a process for producing a connector copper alloy which comprises the steps of:
melting an alloy that contains 23-28 wt % Zn and 0.3-1.8 wt % Sn while satisfying the following relation (1), with the balance being Cu and incidental impurities:
6.0xe2x89xa60.25X+Yxe2x89xa68.5xe2x80x83xe2x80x83(1) 
where X is the addition of Zn (in wt %) and Y is the addition of Sn (in wt %);
cooling the melt from the liquidus line to 600xc2x0 C. at a rate of at least 50xc2x0 C./min;
subsequently hot rolling the resulting ingot at an elevated temperature of 900xc2x0 C. or below;
repeating the process of cold rolling and annealing in a temperature range of 300-650xc2x0 C. until the as-annealed rolled strip has a crystal grain size of no more than 25 xcexcm; and
further performing cold rolling for a reduction ratio of at least 30% and cold annealing at 450xc2x0 C. or below so that the rolled strip has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young""s modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas it has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.
In either of the processes described above, said copper alloy may further contain at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-3 wt % Si, 0.01-5 wt % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.0-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of said elements being 0.01-5 wt %, provided that S is present in an amount of up to 30 ppm.
To produce the connector copper alloy of the invention in rolled strip form, a molten copper alloy adjusted to have the desired composition is first poured into a mold, where it is cooled from the liquidus line to 600xc2x0 C. at a rate of at least 50xc2x0 C./min to ensure that there will be no segregation of Zn and Sn in the resulting ingot. The ingot is then hot rolled at an elevated temperature not higher than 900xc2x0 C., say, at about 800xc2x0 C. and subsequently quenched to produce a hot rolled strip having a homogeneous structure of moderately sized crystal grains. Thereafter, the strip is cold rolled and annealed at a temperature of 300-650xc2x0 C., with the process of cold rolling and annealing being repeated the necessary times, so that the size of crystal grains in the rolled strip is no more than 25 xcexcm. Preferably, the rolled strip is further subjected to cold rolling for a reduction ratio of at least 30% and low-temperature annealing at 450xc2x0 C. or below to control the size of the crystal grains so that it has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, an electrical conductivity of at least 20% IACS, a Young""s modulus of no more than 120 kN/mm2 and a percent stress relaxation of no more than 20% in the direction where it was wrought whereas it has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction.
The present invention will now be described in greater detail.
[Criticality of the Proportions of Alloying Elements]
Zn: Zinc (Zn) is desirably added in large amounts since it contributes to enhanced strength and spring quality and is available at a lower price than Cu. If its addition exceeds 28 wt %, extensive intergranular segregation occurs in the presence of Sn, causing significant drop in hot workability. Also unfavorably affected are cold workability and resistance to corrosion, and resistance to stress corrosion cracking. Platability and solderability which are sensitive to moisture and heat are also deteriorated. If the addition of Zn is smaller than 23 wt %, strength and spring quality that are typified by 0.2% yield strength and tensile strength are insufficient and Young""s modulus increases. What is more, if scrap that was surface treated with Sn is used as the material to be melted, the resulting melt will occlude an increased amount of hydrogen gas to produce an ingot in which blow holes are highly likely to occur. Since Zn is an inexpensive element, using less than 23 wt % of it is an economical disadvantage. For these reasons, the Zn content is specified to range from 23 to 28 wt %. A preferred range is from 24 to 27 wt %. The small range for the Zn content is one of the basic requirements of the present invention.
Sn: Tin (Sn) has the advantage that it need be used in a very small amount to be effective in improving mechanical characteristics such as strength and elasticity typified by 0.2% yield strength and tensile strength without increasing Young""s modulus. Since Sn is an expensive element, materials having a surface Sn coat such as tin plating can be put into a recycle path and this is another reason why incorporating Sn is preferred. However, if the Sn content increases, electrical conductivity drops sharply and extensive intergranular segregation occurs in the presence of Zn, causing significant drop in hot workability. In order to ensure the desired hot workability and an electrical conductivity of at least 20% IACS, the addition of Sn should not exceed 1.8 wt %. If the addition of Sn is less than 0.3 wt %, there will be no improvement in mechanical characteristics and chips or the like that result from the pressing of tin-plated or otherwise tin-coated scrap are difficult to use as the material to be melted. Therefore, the content of Sn is specified to range from 0.3 to 1.8 wt %, preferably from 0.6 to 1.4 wt %.
If Zn and Sn are contained in the amounts specified above and if they satisfy the following relation (1), preferably the following relation (2), the Zn- and Sn-rich phases that precipitate at grain boundaries under high temperature as when casting or hot rolling is performed can be effectively controlled to produce a copper alloy that has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young""s modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought, that has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction, and that also has the characteristics required for use as connector materials, as exemplified by resistance to corrosion, resistance to stress corrosion cracking (having a cracking life in ammonia vapor which is at least three times the value for class 1 brass), anti-stress relaxation characteristics (the percent stress relaxation at 150xc2x0 C. being no more than one half the value for class 1 brass and comparable to phosphor bronze), and efficient punching on a press:
6.0xe2x89xa60.25X+Yxe2x89xa68.5xe2x80x83xe2x80x83(1) 
6.4xe2x89xa60.25X+Yxe2x89xa68.0xe2x80x83xe2x80x83(2) 
where X is the addition of Zn (in wt %) and Y is the addition of Sn (in wt %).
The content of S as an impurity is desirably held to a minimum. Even a small amount of S will markedly reduce the working capacity, or deformability, in hot rolling. Two typical sources for the entrance of S is scrap that has been plated with tin in a sulfate bath and oils for working such as pressing; controlling the value of S content is effective for preventing cracking in the process of hot rolling. In order to have this effect come into being, S should not be present in an amount greater than 30 ppm, preferably no more than 15 ppm.
Besides Zn and Sn, a third alloying element may be added and it is at least one element selected from the group consisting of 0.01-3 wt % Fe, 0.01-5 wt % Ni, 0.01-3 wt % Co, 0.01-3 wt % Ti, 0.01-2 wt % Mg, 0.01-2 wt % Zr, 0.01-1 wt % Ca, 0.01-3 wt % Si, 0.01-5 w % Mn, 0.01-3 wt % Cd, 0.01-5 wt % Al, 0.01-3 wt % Pb, 0.01-3 wt % Bi, 0.01-3 wt % Be, 0.01-1 wt % Te, 0.01-3 wt % Y, 0.01-3 wt % La, 0.01-3 wt % Cr, 0.01-3 wt % Ce, 0.01-5 wt % Au, 0.01-5 wt % Ag and 0.005-0.5 wt % P, with the sum of the contents of these elements being 0.01-5 wt %.
These elements can enhance strength without substantial deterioration in electrical conductivity, Young""s modulus and machinability. If the ranges for the contents of the respective elements are not observed, the stated effect is not attained or, alternatively, disadvantages will result in various aspects such as hot workability, cold workability, press formability, electrical conductivity, Young""s modulus and cost.
[Criticality for Manufacturing Conditions]
The first step in the process of the present invention for producing hot rolled, copper alloy strips is melting the copper alloy of the invention and casting the melt into an ingot. If scrap having a surface Sn coat, in particular chips resulting from punching on a press, are to be melted, a preliminary heat treatment is preferably performed in air atmosphere or an inert atmosphere at a temperature of 300-600xc2x0 C. for 0.5-24 hours. If the temperature is below 300xc2x0 C. the pressing oil adhering to the chips is not completely burnt; what is more, the moisture that has been absorbed during storage is not fully dried and if the melting step is subsequently initiated by rapid temperature elevation, the moisture is decomposed to evolve hydrogen gas which is taken up by the melt to generate blow holes.
If the melting is done at a temperature higher than 600xc2x0 C., oxidation proceeds so rapidly as to induce dross formation. If dross forms, the melt becomes viscous and the efficiency of the casting operation decreases. Therefore, the temperature for the preliminary heat treatment of the copper alloy to be melted is specified to lie between 300 and 600xc2x0 C. if this heat treatment lasts for less than 0.5 hours, combustion of the pressing oil and drying of the moisture are accomplished only incompletely. If the time of the heat treatment is longer than 24 hours, the parent metal Cu diffuses in the Sn surface coat, where it oxidizes to form a Cuxe2x80x94Snxe2x80x94O system oxide that is not only a dross former but also an economic bottleneck. Therefore, the time of the preliminary heat treatment of the copper alloy is specified to lie between 0.5 and 24 hours. The preliminary heat treatment will bring about satisfactory results if it is performed in air atmosphere but providing an inert gas seal is preferred for the purpose of preventing oxidation. However, some disadvantage will result from the use of a reducing gas since at elevated temperature, the moisture decomposes to evolve hydrogen gas that is taken up by the melt to diffuse in it.
After melting the copper alloy, it is desirably cast by the continuous process which may be either vertical or horizontal, except that the melt is cooled from the liquidus line to 600xc2x0 C. at a rate of at least 50xc2x0 C./min. If the cooling rate is less than 50xc2x0 C./min, segregation of Zn and Sn occurs at grain boundaries and the efficiency of the subsequent hot working step decreases to lower the yield. The temperature range over which the cooling rate should be held not lower than 50xc2x0 C./min may be between the liquidus line and 600xc2x0 C. There is no sense of controlling the cooling rate at temperatures higher than the liquidus line; below 600xc2x0 C., the duration of cooling in the casting process is insufficient to cause excess segregation of Zn and Sn at grain boundaries.
After casting the melt into an ingot, hot rolling is performed under heating at a temperature not higher than 900xc2x0 C. Above 900xc2x0 C., intergranular segregation of Zn and Sn causes hot cracking which, in turn, leads to a lower yield. By performing hot rolling at temperatures of 900xc2x0 C. and below, not only the microsegregations that occurred during the casting step but also the cast structure will disappear and the resulting rolled strip has a homogeneous structure even if it contains Zn and Sn in the amounts defined for the copper alloy according to the first aspect of the invention. Preferably, hot rolling is performed at a temperature of 870xc2x0 C. or below. The crystal grains in the hot rolled strip are desirably sized to 35 xcexcm or less. If the crystal grain size exceeds 35 xcexcm, the latitude in control over the reduction ratio for the subsequent cold rolling and the conditions for the annealing that follow is so small that the slightest departure may potentially produce mixed crystal grains, leading to deteriorated characteristics.
After hot rolling, the surface of the strip may be planed as required. Subsequently, cold rolling and annealing in the temperature range of 300-650xc2x0 C. are repeated until the crystals in the as-annealed material have a grain size of no more than 25 xcexcm. Below 300xc2x0 C., it takes an uneconomically prolonged time to control the crystal grains; above 650xc2x0 C., the crystal grains become coarse in a short time. If the size of the crystal grains in the as-annealed material exceeds 25 xcexcm, mechanical characteristics, in particular 0.2% yield strength, or workability deteriorates. Preferably, the crystal grain size is reduced to 15 xcexcm or below, more preferably 10 xcexcm or below.
The thus annealed material is subjected to cold rolling for a reduction ratio of at least 30% and cold annealing at 450xc2x0 C. or below so as to produce a copper alloy that has a 0.2% yield strength of at least 600 N/mm2, a tensile strength of at least 650 N/mm2, a Young""s modulus of no more than 120 kN/mm2, an electrical conductivity of at least 20% IACS and a percent stress relaxation of no more than 20% in the direction where said alloy was wrought whereas it has a 0.2% yield strength of at least 650 N/mm2, a tensile strength of at least 700 N/mm2 and a Young""s modulus of no more than 130 kN/mm2 in a direction perpendicular to said first direction. If the reduction ratio in cold rolling is less than 30%, the improvement in strength that is achieved by work hardening is insufficient to achieve the desired improvement in mechanical characteristics. The reduction ratio is preferably at least 60%. Low-temperature annealing is necessary to improve 0.2% yield strength, tensile strength, spring limit value and anti-stress relaxation characteristics. Beyond 450xc2x0 C., so large a heat capacity is applied that the work softens in a short time. Another difficulty is that variations in the characteristics of the work are prone to occur in both a batch and a continuous system. Hence, cold annealing should be performed at temperatures not higher than 450xc2x0 C.
The thus obtained material may optionally be subjected to surface treatments to provide a Cu undercoat 0.3-2.0 xcexcm thick and a Sn surface film 0.5-5.0 xcexcm thick before it is put to service. If the Cu undercoat is thinner than 0.3 xcexcm, it is by no means effective in preventing the Zn in the alloy from diffusing into the Sn surface coat and to the surface where it is oxidized to increase contact resistance while reducing solderability. If the Cu undercoat is thicker than 2.0 xcexcm, its effect is saturated and there is no economic advantage. The Cu undercoat need not be solely made of pure copper but may be composed of a copper alloy such as Cuxe2x80x94Fe or Cuxe2x80x94Ni.
If the Sn surface coat is thinner than 0.5 xcexcm, the desired resistance to corrosion, particularly to hydrogen sulfide, is not obtained. If the Sn surface coat is thicker than 5.0 xcexcm, its effect is saturated and an economic disadvantage will simply result. To secure uniformity in film thickness and economy, the surface treatments for providing the Cu undercoat and the Sn surface coat are preferably performed by electroplating. The Sn surface coat may be reflowed to improve its gloss. This treatment is also effective as a means of preventing Sn whiskers.
The thus treated material is pressed into electric terminals, which may subsequently be heat treated at a temperature of 100-280xc2x0 C. for a duration of 1-180 minutes. This heat treatment is not only effective for improving on the spring limit value and anti-stress relaxation characteristics that have deteriorated as the result of press working but also instrumental to the prevention of whiskers. Below 100xc2x0 C., these effects of the heat treatment are not fully attained; above 280xc2x0 C., diffusion and subsequent oxidation not only increase the contact resistance but also lower the solderability and workability. If the duration of the heat treatment is shorter than 1 minute, its effects are not fully attained; if it continues longer than 180 minutes, diffusion and subsequent oxidation bring about the unwanted results just mentioned above and, in addition, there is no economic advantage.
The following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting.